A Spin-label Study of the Photosynthetic Bacterium, Rhodospirillum rubrum

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Abstract
The photosynthetic bacterium Rhodospirillum rubrum and chromatophores taken from it have been studied with hydrophobic spin-labels. Stearic labels and lecithin lebels bind to the chromatophores and give ESR spectra consisting of two components, one corresponding to labels situated in the lipid bilayer portion of the membrane and the other due to labels directly or indirectly associated with the membrane proteins. The cytoplasmic fluid obtained by cell rupture shows a strong reducing activity in the dark for nitroxide spin-labels in aqueous solution as well as in chromatophore membranes. The reducing activity of the fluid treated in a variety of ways has been investigated. Both the proteins and some low molecular weight compounds in the fluid are necessary for the activity. NADH or NAD++malate can replace the low molecular compounds. The protein components catalyze the reduction of nitroxides by the low molecular compounds, probably NAD++ malate, in the cytoplasmic fluid. The reduction of membrane-bound nitroxides results from some redox interaction of the reductase system with proteins in the membrane. The reduced nitroxides in chromatophore membranes are rapidly oxidized on irradiation at λ>660nm. The photo-oxidation probably occurs coupled with the electron transport system of the chromatophores. The stationary signal level of membrane-bound labels under illumination is governed largely by a balance between dark-reduction by the reductase system and photo-oxidation by chromatophores and reflects the redox potential of the photosynthetic system in R. rubrum cells.

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